The E583C supports 3G/UMTS at 900 and 2100 MHz and some sources also mention 1900 MHz.
i-mobile 3GX is a MVNO on the TOT3G network which runs its 3G/UMTS network at 2100 MHz. However their 3G/UMTS network's coverage is pretty limited:
iLocation (check "3G signal" for the coverage overlay)
With the E583C you cannot access DTAC's 3G/UMTS network since it is operated at 850 MHz. You would only be ablte to access DTAC's 2G/GSM network which runs at 1800 MHz.
Therefore you should only use i-mobile 3GX/TOT3G or AIS, which may provide better coverage than TOT3G/imobile 3GX:
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